The present invention relates generally to the measurement of transmission system parameters, and more particularly to a measurement test set and method for in-service measurement of carrier phase noise or carrier phase jitter.
Phase noise is a common phase distortion existing in transmitters and receivers of transmission systems. It is caused by phase jitter in the transmitter's and/or the receiver's local oscillators. The existence of phase noise can severely degrade the signal quality of a digitally transmitted RF signal. Typically the distortion present in the received signal are a mixture of linear and nonlinear magnitude errors, linear and nonlinear phase errors, additive noise, and phase noise. To monitor the quality of the transmitted signal and to trouble-shoot a degraded transmission system, accurate measurement of phase noise is very useful. However, the phase noise in the received signal is often combined with above mentioned linear and nonlinear phase errors, phase-induced additive noise errors. Due to the difficulty of separating the various phase errors, phase noise is traditionally measured in an out-of-service mode. An out-of-service mode not only requires removing the carrier's modulation, but also causes the possible loss of the phase noise characteristics caused by the presence of the digital signal as well as other distortions.
In an 8 level Vestigial Side Band (8-VSB) digital modulation system, for example, the overall system response of the combined transmitter and receiver corresponds to a raised cosine filter to avoid system generated intersymbol interference. The system response is implemented with nominally identical root raised cosine filters in the transmitter and in the receiver.
The information bearing digital data stream is randomized for spectrum spreading over the bandwidth of the frequency channel. The randomized data is forward-error-corrected (FEC) coded and interleaved. The data is trellis encoded as an 8-level (3-bit) one dimensional constellation with the outputs of the trellis coder referred to as symbols that are one of eight symmetric odd-valued integer levels from -7 to +7 units. To aid synchronization in low signal to noise and/or high multipath situations, segment and field syncs are inserted in the 10.76 Msymbols/sec symbol stream. A small pilot tone is added as well at the carrier frequency generated by offsetting the real or I channel of the complex signal containing the data and the sync pulses by 1.25 units. The offset causes the pilot tone to be in-phase with the I channel carrier frequency. At the transmitter, the composite signal passes through a root raised cosine filter and modulates an intermediate frequency carrier signal which is up-converted to an RF frequency for transmission at the desired channel frequency. Alternately, the composite signal may be used to directly modulate the RF carrier.
Synchronous demodulation may be used to detect the eight constellation decision levels. However, the constellation of the transmitted symbols may change due to the previously mentioned distortions. For example, the constellation may bend or stretch to form certain curvatures due to the nonlinear distortions in the system. Therefore, for measurement purposes, attempting to determine the transmitted symbols from the received signal in the constellation space would not be reliable using the conventional slicing method.
What is needed is a method and apparatus for accurate in-service measurement of transmitter phase noise of a received signal, where the signal has a mixture of linear distortions, nonlinear distortions, phase noise and additive noise present, that uses reliable constellation decision levels to estimate transmitted digital symbols and preserves original spectral information of the received signal.